1. Field of the Disclosure
The present disclosure relates to a technique for preventing an axial gap-type electric motor, which arranges a stator and a disk-shaped rotor to be opposite each other along a direction of an axis of an output shaft, from being resonated by a surface vibration.
2. Description of the Related Art
It is known in the art that when a rotational speed of a rotor of a electrical motor reaches a characteristic resonant frequency of the electrical motor, the rotor vibrates, which causes the electrical motor to resonate.
In this regard, Japanese Laid-Open Patent Publication No. (Hei) 8-107650 (“Patent Document”) discloses a technique for reducing the resonance of the electrical motor.
The electrical motor disclosed in the above reference changes installation rigidity by adjusting a fastening force of an installation rigidity adjusting bolt, which installs a heat exchanger in a frame of the electrical motor. This changes a characteristic frequency of the entire electrical motor. The technique disclosed in said reference is directed to enable quiet operation of the electric motor by avoiding resonation, even if the installation rigidity of the electrical motor differs from the evaluation conducted at the time of shipment from a factory.
However, the characteristic frequency of the rotor itself cannot be even by the conventional electrical motor. Particularly, in an axial gap-type electrical motor having a disk-shaped rotor, the thickness of the rotor is thin compared to its radius. Thus, the rigidity of the rotor is low and a surface vibration, which is when the ends of the rotor are bent or distorted with respect to a virtual plane perpendicular to the axis of the output shaft and the ends rapidly oscillate from one side of the virtual plane to another, tends to occur.
When the rotational speed of the rotor conforms to the characteristic frequency of the rotor itself, the resonance of the rotor is considerably increased, thereby disturbing the quiet operation of the electrical motor.
Further, a reinforcing member such as a rib may be installed in the rotor. However, the reinforcing member may inhibit the functionality of a magnetic circuit of an electromagnetic force for driving the rotor. The rib may also increase the volume of the rotor itself, and thus, the density of a magnetic flux along the rotor may be weakened, thereby decreasing the rotor output torque.